Apparatus and method for medium access control in wireless communication networks

ABSTRACT

A method includes identifying a white space at a first wireless node and selecting a channel in the identified white space. The white space includes at least one frequency or frequency band not in use (like a licensed frequency or frequency band). The method also includes identifying at the first wireless node a channel access factor for each of multiple wireless nodes including the first wireless node. The method further includes determining if the first wireless node has a specified channel access factor. In addition, the method includes transmitting data from the first wireless node on the channel when the first wireless node has the specified channel access factor. The channel access factors can be identified and the determination whether the first wireless node has the specified channel access factor can be performed without using control signals transmitted between the wireless nodes. The channel access factor could represent a hash function value.

TECHNICAL FIELD

This disclosure relates generally to wireless communication networks.More specifically, this disclosure relates to an apparatus and methodfor medium access control in wireless communication networks.

BACKGROUND

Radio frequencies often represent very valuable (and sometime scarce)resources. Radio frequencies can often be divided into two generalcategories, namely licensed and unlicensed. Licensed frequency bandsrepresent frequencies whose use is often tightly controlled, whileunlicensed frequency bands are available for use by many entities withlittle or no regulation. Unlicensed frequency bands include theso-called “Industrial, Scientific, and Medical” (ISM) bands.

In recent years, the unlicensed frequency bands have become quitecrowded. This is due to many factors, including the proliferation ofwireless devices in the ISM bands, the explosive growth of 802.11popularity, and expected city-wide WiFi and other mesh networks. On theother hand, many licensed spectrums are extremely underutilized. Somemeasurements have shown that only five percent of the licensed spectrumfrom 30 MHz to 30 GHz is actively used in the United States.

Because of this, recent interest has been shown in permitting the use ofunlicensed devices in the licensed frequency bands. One approach is toallow unlicensed devices to operate using licensed frequencies orfrequency bands as long as licensed users (called “primary” users) arenot operating on the licensed frequencies or frequency bands. Thespectrums or portions thereof that are not in active use by primaryusers are called “white spaces.”

Cognitive radios are one emerging and quite promising technique tofacilitate the use of white spaces. Cognitive radios are designed tochange their transmission and reception parameters to avoid interferencewith other devices. The use of cognitive radios often requires anefficient Medium Access Control (MAC) scheme to take advantage ofpotentially dynamic white spaces. However, conventional MAC schemes aretypically deterministic and therefore require precise coordination(control signal exchanges) among users. In addition to large overheads,these types of schemes are often highly fragile and unreliable inreal-world operations.

SUMMARY

This disclosure provides an apparatus and method for medium accesscontrol in wireless communication networks.

In a first embodiment, a method includes identifying a white space at afirst wireless node and selecting a channel in the identified whitespace. The white space includes at least one frequency or frequency bandnot in use. The method also includes identifying at the first wirelessnode a channel access factor for each of multiple wireless nodesincluding the first wireless node. The method further includesdetermining if the first wireless node has a specified channel accessfactor. In addition, the method includes transmitting data from thefirst wireless node on the channel when the first wireless node has thespecified channel access factor.

In particular embodiments, identifying the channel access factor foreach of the multiple wireless nodes and determining if the firstwireless node has the specified channel access factor are performedwithout using control signals transmitted between the wireless nodes.

In other particular embodiments, the channel includes a frequency slotand a time slot. Also, the channel access factor includes a hashfunction value determined using a hash function. The hash function isbased on a wireless node identifier, the frequency slot, and/or the timeslot. The hash function could also be based on a wireless node weight.The specified channel access factor could include a highest hashfunction value.

In yet other particular embodiments, the method also includes notifyingone or more receiving wireless nodes that the first wireless node willtransmit the data on the channel.

In still other particular embodiments, the method also includestransmitting information associated with the identified white space toat least one other wireless node. The method further includes receivinginformation associated with at least one additional identified whitespace from the at least one other wireless node. The informationassociated with the identified white space could be transmitted toone-hop and two-hop neighbors of the first wireless node. Each one-hopneighbor includes a wireless node directly communicating with the firstwireless node, and each two-hop neighbor includes a wireless nodedirectly communicating with a one-hop neighbor.

In additional particular embodiments, the at least one frequency orfrequency band not in use includes at least one licensed frequency orfrequency band.

In a second embodiment, an apparatus includes a spectrum sensing unitconfigured to identify a white space, where the white space includes atleast one frequency or frequency band not in use. The apparatus alsoincludes a channel selection unit configured to identify a channelaccess factor for each of multiple wireless nodes (where the wirelessnodes include the apparatus). The channel selection unit is alsoconfigured to determine if the apparatus has a specified channel accessfactor. In addition, the apparatus includes a control unit configured tocontrol transmission of data from the apparatus on the channel dependingon whether the apparatus has the specified channel access factor.

In a third embodiment, a computer program is embodied on a computerreadable medium. The computer program includes computer readable programcode for identifying a white space at a first wireless node and computerreadable program code for selecting a channel in the identified whitespace. The white space includes at least one frequency or frequency bandnot in use. The computer program also includes computer readable programcode for identifying at the first wireless node a channel access factorfor each of multiple wireless nodes including the first wireless node.The computer program further includes computer readable program code fordetermining if the first wireless node has a specified channel accessfactor. In addition, the computer program includes computer readableprogram code for initiating transmission of data from the first wirelessnode on the channel when the first wireless node has the specifiedchannel access factor.

Other technical features may be readily apparent to one skilled in theart from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following description, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates an example wireless communication network inaccordance with this disclosure;

FIG. 2 illustrates an example process control system in accordance withthis disclosure;

FIG. 3 illustrates an example wireless node in a wireless communicationnetwork in accordance with this disclosure; and

FIG. 4 illustrates an example method for medium access control in awireless communication network in accordance with this disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 4, discussed below, and the various embodiments used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the invention may be implemented inany type of suitably arranged device or system.

FIG. 1 illustrates an example wireless communication network 100 inaccordance with this disclosure. The embodiment of the wirelesscommunication network 100 shown in FIG. 1 is for illustration only.Other embodiments of the wireless communication network 100 could beused without departing from the scope of this disclosure.

As shown in FIG. 1, the wireless communication network 100 includesmultiple wireless nodes 102 a-102 e and 104 a-104 b. The wireless nodes102 a-102 e and 104 a-104 b generally represent devices or systems thattransmit and/or receive data wirelessly. For example, the wireless nodes102 a-102 e and 104 a-104 b could represent wireless access points(WAPs), wireless routers, base stations, or other devices or systemsthat communicate wirelessly. Each of the wireless nodes 102 a-102 e and104 a-104 b includes any suitable structure(s) for communicatingwirelessly. As a particular example, at least some of the wireless nodes102 a-102 e and 104 a-104 b could represent cognitive radios, such asSoftware Defined Radios (SDRs) (like those implemented using the ETTUSSDR platform).

The wireless nodes 102 a-102 e and 104 a-104 b could transmit andreceive any suitable wireless signals. For example, the wireless nodes102 a-102 e and 104 a-104 b could use radio frequency (RF) signals tocommunicate. Each of the wireless nodes 102 a-102 e and 104 a-104 b alsotypically communicates over one or more channels. The channels couldrepresent frequency-time slots, where a wireless node communicates in aparticular time slot using a particular frequency.

Wireless communications by the wireless nodes 102 a-102 e createinterference regions 106 a-106 e around the wireless nodes 102 a-102 e.Similarly, wireless communications by the wireless nodes 104 a-104 bcreate interference regions 108 a-108 b around the wireless nodes 104a-104 b. Each of the interference regions 106 a-106 e and 108 a-108 brepresents an area where one wireless node's communications caninterfere with the communications of other wireless nodes. Althoughshown as being circular in FIG. 1, each of the interference regions 106a-106 e and 108 a-108 b could have any other shape (such as a shapebased on the placement of the wireless node and the environment aroundthe wireless node). In FIG. 1, each of the wireless nodes 102 a-102 eand 104 a-104 b is located in the interference region of at least oneother wireless node. It may be noted, however, that one or more of thewireless nodes could be located outside all other wireless nodes'interference regions.

In some embodiments, the wireless nodes 102 a-102 e and 104 a-104 brepresent different licensed and unlicensed devices or systems (withrespect to the frequencies or frequency bands used for communications).For example, the wireless nodes 104 a-104 b could be licensed to use oneor more licensed frequencies or frequency bands, while the wirelessnodes 102 a-102 e may not be licensed to use the one or more licensedfrequencies or frequency bands. In this case, the wireless nodes 104a-104 b represent “primary users,” meaning they have priority to use thelicensed frequencies or frequency bands. The wireless nodes 102 a-102 erepresent “secondary users,” meaning they are required to avoidinterfering with the primary users' use of the licensed frequencies orfrequency bands.

The wireless nodes 102 a-102 e therefore engage in various operations toidentify one or more channels that can be used for wirelesscommunications. For example, the wireless nodes 102 a-102 e can identifywhite spaces that are not in active use by any primary users. Thewireless nodes 102 a-102 e can then select and use channels (such asfrequency-time slots) in the identified white spaces. This allows thewireless nodes 102 a-102 e to select channels that do not interfere withthe communications involving the wireless nodes 104 a-104 b.

Conventional systems often require a large number of control signals tobe exchanged between wireless nodes so the wireless nodes can selectchannels for communications. This can lead to large overheads,reductions in network throughput, unreliable network behaviors, or otherproblems. As described in more detail below, once the white spaces areidentified, the wireless nodes 102 a-102 e implement a function (such asa hash function) that allows the wireless nodes 102 a-102 e to identifywhich nodes can use channels in the white spaces. Moreover, the wirelessnodes 102 a-102 e can implement this function in a way that reduces theneed for control signals to be exchanged between the wireless nodes 102a-102 e. This can reduce overhead, increase throughput, and increasereliability in the wireless communication network 100.

Although FIG. 1 illustrates one example of a wireless communicationnetwork 100, various changes may be made to FIG. 1. For example, thewireless communication network 100 could include any number of licensedand unlicensed wireless nodes. Also, the licensed and unlicensedwireless nodes could have any suitable configuration. In addition, theinterference regions of the wireless nodes could have any suitable sizesand shapes.

FIG. 2 illustrates an example process control system 200 in accordancewith this disclosure. In particular, FIG. 2 illustrates a processcontrol system 200 having unlicensed wireless nodes that can operateusing licensed frequencies or frequency bands. The embodiment of theprocess control system 200 shown in FIG. 2 is for illustration only.Other embodiments of the process control system 200 could be usedwithout departing from the scope of this disclosure.

As shown in FIG. 2, the process control system 200 includes one or moreprocess elements 202. The process elements 202 represent components in aprocess system that may perform any of a wide variety of functions. Forexample, the process elements 202 could represent sensors, actuators, orany other or additional industrial equipment in a processingenvironment. Each of the process elements 202 includes any suitablestructure for performing one or more functions in a process system.Also, a “process system” may generally represent any system or portionthereof configured to process one or more products or other materials insome manner.

A controller 204 is coupled to the process elements 202. The controller204 controls the operation of one or more of the process elements 202.For example, the controller 204 could receive information associatedwith the process system, such as by receiving sensor measurements fromsome of the process elements 202. The controller 204 could use thisinformation to provide control signals to others of the process elements202, thereby adjusting the operation of those process elements 202. Thecontroller 204 includes any hardware, software, firmware, or combinationthereof for controlling one or more process elements 202. The controller204 could, for example, represent a computing device executing aMICROSOFT WINDOWS operating system.

A network 206 facilitates communication between various components inthe system 200. For example, the network 206 may communicate InternetProtocol (IP) packets, frame relay frames, Asynchronous Transfer Mode(ATM) cells, or other suitable information between network addresses.The network 206 may include one or more local area networks (LANs),metropolitan area networks (MANs), wide area networks (WANs), all or aportion of a global network such as the Internet, or any othercommunication system or systems at one or more locations.

As shown in FIG. 2, the process control system 200 also includes one ormore wireless networks for communicating with wireless sensors or otherwireless devices. In this example, a wireless network (such as a meshnetwork) is formed using infrastructure nodes (“I nodes”) 208 a-208 e,leaf nodes 210 a-210 e, and a gateway infrastructure node 212.

The infrastructure nodes 208 a-208 e and the leaf nodes 210 a-210 eengage in wireless communications with each other. For example, theinfrastructure nodes 208 a-208 e may receive data transmitted over thenetwork 206 (via the gateway infrastructure node 212) and wirelesslycommunicate the data to the leaf nodes 210 a-210 e. Similarly, the leafnodes 210 a-210 e may wirelessly communicate data to the infrastructurenodes 208 a-208 e for forwarding to the network 206 (via the gatewayinfrastructure node 212). In addition, the infrastructure nodes 208a-208 e may wirelessly exchange data with one another. In this way, thenodes 208 a-208 e and 210 a-210 e form a wireless network capable ofproviding wireless coverage to a specified area, such as in a largeindustrial complex.

In this example, the nodes 208 a-208 e and 210 a-210 e are divided intoinfrastructure nodes and leaf nodes. The infrastructure nodes 208 a-208e typically represent line-powered devices, meaning these nodes receiveoperating power from an external source. As a result, these nodes 208a-208 e are typically not limited in their operations since they neednot minimize power consumption to increase the operational life of theirinternal power supplies. On the other hand, the leaf nodes 210 a-210 etypically represent devices powered by local power supplies, such asnodes that receive operating power from internal batteries or otherinternal power supplies. Because of this, these nodes 210 a-210 e areoften more limited in their operations in order to help preserve theoperational life of their internal power supplies. These nodes 210 a-210e also routinely need to have their internal batteries or other internalpower supplies replaced in order to remain in operation.

Each of the nodes 208 a-208 e and 210 a-210 e includes any suitablestructure facilitating wireless communications, such as an RFtransceiver. Each of the nodes 208 a-208 e and 210 a-210 e could alsoinclude other functionality, such as functionality for generating orusing data communicated over the wireless network. For example, the leafnodes 210 a-210 e could represent wireless sensors in an industrialfacility, where the sensors are used to measure various characteristicswithin the facility. These sensors could collect sensor readings andcommunicate the sensor readings to the controller 204 via the gatewayinfrastructure node 212. The leaf nodes 210 a-210 e could also representactuators that can receive control signals from the controller 204 andadjust the operation of the industrial facility. In this way, the leafnodes 210 a-210 e may include or operate in a similar manner as theprocess elements 202 that are physically connected to the controller204. The leaf nodes 210 a-210 e could further represent handheld userdevices (such as INTELATRAC devices from HONEYWELL INTERNATIONAL INC.),mobile stations, programmable logic controllers (PLCs), or any other oradditional devices.

The gateway infrastructure node 212 communicates wirelessly with,transmits data to, and receives data from one or more infrastructurenodes 208 a-208 e and possibly one or more leaf nodes 210 a-210 e. Thegateway infrastructure node 212 also converts data between theprotocol(s) used by the network 206 and the protocol(s) used by thenodes 208 a-208 e and 210 a-210 e. For example, the gatewayinfrastructure node 212 could convert Ethernet-formatted data(transported over the network 206) into a wireless protocol format (suchas an IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.15.3, 802.15.4, or802.16 protocol format) used by the nodes 208 a-208 e and 210 a-210 e.The gateway infrastructure node 212 could also convert data receivedfrom one or more of the nodes 208 a-208 e and 210 a-210 e intoEthernet-formatted data for transmission over the network 206. Inaddition, the gateway infrastructure node 212 could support variousfunctions, such as network creation and security, used to create andmaintain a wireless network. The gateway infrastructure node 212includes any suitable structure for facilitating communication betweencomponents or networks using different protocols.

In particular embodiments, the leaf nodes 210 a-210 e can include802.15.4-based low data-rate sensors and 802.11-based high data-ratedevices, and the various nodes in FIG. 2 form a mesh networkcommunicating at 2.4 GHz or 5.8 GHz. Also, in particular embodiments,data can be injected into the wireless mesh network through theinfrastructure nodes, thus providing versatile, multifunctional,plant-wide coverage for wireless sensing, asset location tracking,personnel tracking, wireless communications, and any other or additionalfunctionality as desired.

In this example, a wireless configuration and OLE for Process Control(OPC) server 214 can be used to configure and control various aspects ofthe process control system 200. For example, the server 214 could beused to configure the operation of the infrastructure nodes 208 a-208 eand the gateway node 212. The server 214 could also be used to supportsecurity in the process control system 200. For instance, the server 214could distribute cryptographic keys or other security data to variouscomponents in the process control system 200, such as to the nodes 208a-208 e, 210 a-210 e, and 212. The server 214 includes any hardware,software, firmware, or combination thereof for configuring wirelessnetworks and providing security information.

In one aspect of operation, the nodes 208 a-208 e, 210 a-210 e, and 212can communicate using wireless channels, such as frequency-time slots.The frequencies or frequency bands used by the nodes 208 a-208 e, 210a-210 e, and 212 can include one or more licensed frequencies orfrequency bands that are also used by at least one primary user 216. Theprimary users 216 represent devices or systems licensed to communicateusing the one or more licensed frequencies or frequency bands. Theprimary users 216 are therefore entitled to use the one or more licensedfrequencies or frequency bands and have priority over the nodes 208a-208 e, 210 a-210 e, and 212.

As described in more detail below, various ones of the nodes 208 a-208e, 210 a-210 e, and 212 can identify white spaces that are not in activeuse by any primary users 216. Those nodes can then select and usechannels (such as frequency-time slots) in the identified white spaces.In addition, a function (such as a hash function) can be used toidentify which nodes can use the channels in the white spaces to avoidcontention. This can be done in a way that reduces the need for controlsignals to be exchanged between the nodes.

Although FIG. 2 illustrates one example of a process control system 200,various changes may be made to FIG. 2. For example, the process controlsystem 200 could include any number of process elements, controllers,networks (wired or wireless), infrastructure nodes (gateway or other),leaf nodes, and servers. Also, the functional division shown in FIG. 2is for illustration only. Various components in FIG. 2 could becombined, subdivided, or omitted and additional components could beadded according to particular needs. In addition, FIG. 2 illustrates oneexample operational environment where the selection of channels in whitespaces can be performed. This functionality could be used with anysuitable device and in any suitable system (whether or not related to orused for process control).

FIG. 3 illustrates an example wireless node 300 in a wirelesscommunication network in accordance with this disclosure. The wirelessnode 300 could, for example, represent various ones of the wirelessnodes in FIG. 1 or FIG. 2. The embodiment of the wireless node 300 shownin FIG. 3 is for illustration only. Other embodiments of the wirelessnode 300 could be used without departing from the scope of thisdisclosure.

As shown in FIG. 3, the node 300 includes a transceiver 302. Thetransceiver 302 facilitates wireless communications to and from the node300 using wireless signals. For example, the transceiver 302 couldreceive a baseband or intermediate data signal and modulate the signalonto a carrier signal for transmission by an antenna 304. Thetransceiver 302 could also receive a carrier signal from the antenna 304and down-convert the signal into a baseband or intermediate signal. Thetransceiver 302 includes any suitable structure for transmitting and/orreceiving wireless signals. In some embodiments, the transceiver 302represents an RF transceiver, and the antenna 304 represents an RFantenna. The transceiver 302 could use any other suitable wirelesssignals to communicate. Also, the transceiver 302 could represent atransmitter and a separate receiver.

A controller 306 is coupled to the transceiver 302. The controller 306controls the overall operation of the node 300. For example, thecontroller 306 may receive or generate data to be transmittedexternally, and the controller 306 could provide the data to thetransceiver 302 for wireless transmission. The controller 306 could alsoreceive data from the transceiver 302 that was transmitted to the node300 and use the data. As a particular example, the controller 306 couldreceive or generate measurement data associated with an industrialprocess and provide the data to the transceiver 302 for transmission toa process controller. The controller 306 could also receive data fromthe process controller and adjust operation of one or more actuators ina process control system. The controller 306 includes any suitablestructure(s) for controlling operation of the node 300. As particularexamples, the controller 306 could represent a processor,microprocessor, microcontroller, field programmable gate array (FPGA),or other processing or control device.

A memory 308 is coupled to the controller 306. The memory 308 stores anyof a wide variety of information used, collected, or generated by thenode 300. For example, the memory 308 could store various informationused by the node 300 to identify channels in white spaces and todetermine which node has priority to use a channel (such as hashfunctions, node identifiers, and frequency-time slot information). Thememory 308 includes any suitable volatile and/or non-volatile storageand retrieval device or devices.

In this example, the memory 308 can store and the controller 306 canexecute instructions for identifying and selecting channels from whitespaces (which could include licensed frequencies or frequency bands). Asshown in FIG. 3, the node 300 executes, supports, or otherwise includesa spectrum sensing unit 310, a channel selection unit 312, and a controlunit 314. The spectrum sensing unit 310 scans various frequencies orfrequency bands to determine if any other devices (such as primaryusers) are using the frequencies or frequency bands. The channelselection unit 312 uses information from the spectrum sensing unit 310(and possibly from other wireless nodes) to select one or more channelsin the white spaces for use. The control unit 314 uses information fromthe channel selection unit 312 to communicate with other wireless nodesusing the selected channels.

In some embodiments, the identification and selection of wirelesschannels in white spaces could occur as follows. The spectrum sensingunit 310 in the wireless node 300 scans licensed frequencies orfrequency bands and identifies white spaces, which are unused portionsof the licensed spectrum. The identified white spaces can be shared withother wireless nodes, such as other nodes within the interference regionof that wireless node 300. In particular embodiments, a “two-hop”neighborhood could be used when sharing white space information, wherethe node 300 shares its identified white spaces with any nodes withintwo hops of the node 300. In this way, each wireless node can build amap of the white spaces that are available to all nodes in that node'sinterference region. A separate control channel can be reserved in thewireless network for the wireless nodes to announce their identifiedwhite spaces and to identify occupied channels, where all nodes can tuneto this control channel when not transmitting or receiving.

The spectrums in the identified white spaces are divided into channels,such as frequency slots and time slots. When a wireless node 300 needsto transmit data, its selection unit 312 can select a channel in theidentified white spaces, such as by reserving a frequency-time slot foritself and the intended receiver(s). The control unit 314 can theninitiate communications using the identified channel if the wirelessnode 300 has priority to use that identified channel.

To determine which wireless node can use a channel in a white space, thechannel selection unit 312 can implement a scheme that uses a reduced orminimum number of hand-shaking or other control signals for the channelreservation process. This allows the channel selection unit 312 toreduce overhead and other problems experienced by conventional systems.For example, the channel selection units 312 could implement functionsto identify a channel access factor for each wireless node. A channelaccess factor is a value that represents a priority of a wireless nodeto use a particular channel. The channel access factors can be computedfor multiple nodes at each of those nodes. The node with the highestchannel access factor could “win” the particular channel, which allowsthe node to communicate using that particular channel. In this way, thenodes can easily identify which channels can be used by those nodes,with little or no control signals required between the nodes.

As a particular example, channel selection units 312 in multiplewireless nodes can use pseudo-random hash functions to achieve weightedallocations without explicit control signal exchanges. For instance,each wireless node can be identified by or associated with a uniqueidentifier id_(i). A hash function can be distributed to each wirelessnode, such as before that wireless node is deployed in a wirelessnetwork. For each time slot t and for each frequency slot f, the channelselection unit 312 in a wireless node can compute the hash valueH_(i, f, t) for itself and for each node in its interference region. Thehash value H_(i, f, t) for node i can be expressed as:H _(i,f,t) =R(id _(i) ,f,t).  (1)Here, R represents a function performed using the node identifierid_(i), a frequency slot f, and a time slot t. A node id_(i) “wins” oris allowed to communicate using a particular frequency-time slot (f, t)if its hash value H_(i, f, t) is the largest hash value for all nodes.This can be expressed as:

$\begin{matrix}{i_{f,t} = {\arg{\;\;}{\max\limits_{i}{H_{i,f,t}.}}}} & (2)\end{matrix}$If the function R is a uniformly distributed pseudo-random numbergenerator, the above allocation can guarantee equal opportunities forall nodes on all frequency slots.

This scheme can be easily extended to support weighted allocations, suchas to favor wireless nodes with higher data rates, a higher class ofservice, an assigned priority, or more important data. In this weightedscheme, the hash function H could be defined as follows:H _(i,f,t) =R(id _(i) ,f,t)^(1/w).  (3)Here, R can be a uniformly distributed pseudo-random number generator.Also, the probability of a node i winning a frequency-time slot isproportional to its weight w_(i), and this can be expressed as:

$\begin{matrix}{{P\left\lbrack {i_{f,t} = {\arg{\;\;}{\max\limits_{i}H_{i,f,t}}}} \right\rbrack} = {\frac{w_{i}}{\sum\limits_{k}w_{k}}.}} & (4)\end{matrix}$

This contention process can involve computations in wireless nodeswithout any control signal exchanges between those wireless nodes. Thewinner of a frequency-time slot (f, t) can subsequently advertise itsfrequency-time slot over the dedicated control channel. This allows anyreceivers to tune to the right channel at the right time, and it allowsother nodes to remain silent during that particular slot.

Among other things, this provides a contention process that cansignificantly reduce the number of control signals and communicationoverhead, while also improving robustness, spectral utilization, andnetwork throughput. This can be done by reducing or eliminating anydependencies on reliable control signal exchanges. In addition, thisprocess can eliminate the need for a network-wide central controllerthat is responsible for white space construction and allocation. Thiscan improve the scalability of the system while still enabling spectrumspatial reuse.

The spectrum sensing unit 310 includes any hardware, software, firmware,or combination thereof for scanning frequencies or frequency bands todetermine if the frequencies or frequency bands are being used. Thechannel selection unit 312 includes any hardware, software, firmware, orcombination thereof for selecting one or more channels for use. Thecontrol unit 314 includes any hardware, software, firmware, orcombination thereof for controlling the selection and use of channels bythe wireless node 300. As a particular example, these components 310-314could be implemented as a Layer-2 service in software defined radioplatforms for cognitive radio networks.

Although FIG. 3 illustrates one example of a wireless node 300 in awireless communication network, various changes may be made to FIG. 3.For example, various components in FIG. 3 could be combined, subdivided,or omitted and additional components could be added according toparticular needs. Also, while the components 310-314 have been describedas residing in the memory 308 and being executed by the controller 306,the components 310-314 could be implemented in any other suitable manner(such as using physical circuits).

FIG. 4 illustrates an example method 400 for medium access control in awireless communication network in accordance with this disclosure. Theembodiment of the method 400 shown in FIG. 4 is for illustration only.Other embodiments of the method 400 could be used without departing fromthe scope of this disclosure.

One or more spectrums are scanned to identify one or more white spacesat step 402. This could include, for example, the spectrum sensing unit310 in a wireless node 300 scanning one or more licensed spectrums toidentify frequencies or frequency bands that are not currently in use.

The one or more identified white spaces are shared with one or moreother nodes at step 404. This could include, for example, the controlunit 314 providing information defining the one or more identified whitespaces to the transceiver 302 for transmission. The information could betransmitted from that wireless node 300 to any one-hop and two-hopneighbors of that node 300. The one-hop neighbors represent any wirelessnodes directly communicating with the node 300. The two-hop neighborsrepresent any wireless nodes directly communicating with the one-hopneighbors.

One or more white spaces identified by other nodes are received at step406. This could include, for example, the control unit 314 receivingfrom the transceiver 302 information defining white spaces that havebeen identified by its one-hop and two-hop neighbors.

Data to be transmitted is generated or received at step 408. This couldinclude, for example, the wireless node 300 generating data to betransmitted to at least one other wireless node. This could also includethe wireless node 300 receiving the data from an external source.

The wireless node selects a channel at step 410. This could include, forexample, the channel selection unit 312 selecting a frequency-time slot(f, t). The wireless node computes a hash function value for itself andone or more neighboring nodes at step 412. This could include, forexample, the channel selection unit 312 computing a hash function valueH_(i, f, t) using its own identifier and the frequency-time slot (f, t).This could also include the channel selection unit 312 computing a hashfunction value H_(i, f, t) using the identifier of each one-hop ortwo-hop neighbor and the frequency-time slot (f, t).

The wireless node decides if its hash function value is the highestvalue at step 414. This could include, for example, the channelselection unit 312 comparing its own hash function value H_(i, f, t)against the hash function values H_(i, f, t) for its neighboring nodes.In the event of a tie between two or more nodes having equal hashfunction values, a tie-breaker could be used, such as selecting thewireless node with highest identifier value.

If the wireless node does not have the highest hash function value, theprocess returns to step 410, where the wireless node can select anotherchannel and recompute the hash function values. Eventually, the wirelessnode 300 finds a channel where the node has the highest hash functionvalue.

When the wireless node has the highest hash function value, one or morereceiving nodes are notified that the wireless node will transmit dataon the selected channel at step 416. This could include, for example,the control unit 314 in the node 300 initiating a broadcast of a messageon a control channel, where the message indicates that the selectedfrequency-time slot (f, t) belongs to that node 300. This allows anyreceiving node(s) to tune to the frequency-time slot (f, t). The data isthen transmitted to the one or more receiving nodes at step 418. Thiscould include, for example, the control unit 314 initiating a broadcastof the data using the selected frequency-time slot (f, t).

Although FIG. 4 illustrates one example of a method 400 for mediumaccess control in a wireless communication network, various changes maybe made to FIG. 4. For example, while shown as a series of steps,various steps in FIG. 4 could overlap, occur in parallel, occur multipletimes, or occur in a different order. Also, while shown as using hashfunction values, any other suitable channel access factors could beused.

In some embodiments, various functions described above are implementedor supported by a computer program that is formed from computer readableprogram code and that is embodied in a computer readable medium. Thephrase “computer readable program code” includes any type of computercode, including source code, object code, and executable code. Thephrase “computer readable medium” includes any type of medium capable ofbeing accessed by a computer, such as read only memory (ROM), randomaccess memory (RAM), a hard disk drive, a compact disc (CD), a digitalvideo disc or digital versatile disc (DVD), or any other type of medium.

It may be advantageous to set forth definitions of certain words andphrases used throughout this patent document. The term “couple” and itsderivatives refer to any direct or indirect communication between two ormore elements, whether or not those elements are in physical contactwith one another. The terms “transmit,” “receive,” and “communicate,” aswell as derivatives thereof, encompass both direct and indirectcommunication. The terms “include” and “comprise,” as well asderivatives thereof, mean inclusion without limitation. The term “or” isinclusive, meaning and/or. The phrases “associated with” and “associatedtherewith,” as well as derivatives thereof, may mean to include, beincluded within, interconnect with, contain, be contained within,connect to or with, couple to or with, be communicable with, cooperatewith, interleave, juxtapose, be proximate to, be bound to or with, have,have a property of, or the like. The term “controller” means any device,system, or part thereof that controls at least one operation. Acontroller may be implemented in hardware, firmware, software, or somecombination of at least two of the same. The functionality associatedwith any particular controller may be centralized or distributed,whether locally or remotely.

While this disclosure has described certain embodiments and generallyassociated methods, alterations and permutations of these embodimentsand methods will be apparent to those skilled in the art. Accordingly,the above description of example embodiments does not define orconstrain this disclosure. Other changes, substitutions, and alterationsare also possible without departing from the spirit and scope of theinvention, as defined by the following claims.

1. A method comprising: identifying a white space at a first wireless node, the white space comprising at least one frequency or frequency band not in use; selecting a channel in the identified white space; identifying at the first wireless node a channel access factor for each of multiple wireless nodes including the first wireless node; determining if the first wireless node has a specified channel access factor; and transmitting data from the first wireless node on the channel when the first wireless node has the specified channel access factor.
 2. The method of claim 1, wherein identifying the channel access factor for each of the multiple wireless nodes and determining if the first wireless node has the specified channel access factor are performed without using control signals transmitted between the wireless nodes.
 3. The method of claim 1, wherein: the channel comprises a frequency slot and a time slot; and the channel access factor comprises a hash function value determined using a hash function, the hash function based on at least one of: a wireless node identifier, the frequency slot, and the time slot.
 4. The method of claim 3, wherein the hash function is also based on a wireless node weight.
 5. The method of claim 4, wherein the hash function is expressed as: H _(i,f,t) =R(id _(i) ,f,t)^(1/w) ^(i) where H_(i, f, t) represents the hash function value, R represents a uniformly distributed pseudo-random number generator, id_(i) represents the wireless node identifier of node i, f represents the frequency slot, t represents the time slot, and w_(i) represents the weight of node i.
 6. The method of claim 3, wherein the specified channel access factor comprises a highest hash function value.
 7. The method of claim 1, further comprising: notifying one or more receiving wireless nodes that the first wireless node will transmit the data on the channel.
 8. The method of claim 1, further comprising: transmitting information associated with the identified white space to at least one other wireless node; and receiving information associated with at least one additional identified white space from the at least one other wireless node.
 9. The method of claim 8, wherein the information associated with the identified white space is transmitted to one-hop and two-hop neighbors of the first wireless node, each one-hop neighbor comprising a wireless node directly communicating with the first wireless node, each two-hop neighbor comprising a wireless node directly communicating with a one-hop neighbor.
 10. The method of claim 1, wherein the at least one frequency or frequency band not in use comprises at least one licensed frequency or frequency band.
 11. An apparatus comprising: a spectrum sensing unit configured to identify a white space, the white space comprising at least one frequency or frequency band not in use; a channel selection unit configured to identify a channel access factor for each of multiple wireless nodes, the wireless nodes including the apparatus, the channel selection unit also configured to determine if the apparatus has a specified channel access factor; and a control unit configured to control transmission of data from the apparatus on a channel in the identified white space depending on whether the apparatus has the specified channel access factor.
 12. The apparatus of claim 11, wherein the channel selection unit is configured to identify the channel access factor for each of the multiple wireless nodes and to determine if the apparatus has the specified channel access factor without using control signals transmitted between the wireless nodes.
 13. The apparatus of claim 11, wherein: the channel comprises a frequency slot and a time slot; and the channel access factor comprises a hash function value determined using a hash function, the hash function based on at least one of: a wireless node identifier, the frequency slot, the time slot, and a wireless node weight.
 14. The apparatus of claim 13, wherein the specified channel access factor comprises a highest hash function value.
 15. The apparatus of claim 11, further comprising: a transceiver configured to transmit the data on the channel.
 16. The apparatus of claim 11, wherein the spectrum sensing unit, the channel selection unit, and the control unit comprise software instructions stored in a memory and executed by a controller in the apparatus.
 17. A non-transitory computer readable storage medium embodying a computer program, the computer program comprising: computer readable program code for identifying a white space at a first wireless node, the white space comprising at least one frequency or frequency band not in use; computer readable program code for selecting a channel in the identified white space; computer readable program code for identifying at the first wireless node a channel access factor for each of multiple wireless nodes including the first wireless node; computer readable program code for determining if the first wireless node has a specified channel access factor; and computer readable program code for initiating transmission of data from the first wireless node on the channel when the first wireless node has the specified channel access factor.
 18. The computer readable storage medium of claim 17, wherein the computer readable program code for identifying the channel access factor for each of the multiple wireless nodes and the computer readable program code for determining if the first wireless node has the specified channel access factor are not based on control signals transmitted between the wireless nodes.
 19. The computer readable storage medium of claim 17, wherein: the channel comprises a frequency slot and a time slot; and the channel access factor comprises a hash function value determined using a hash function, the hash function based on at least one of: a wireless node identifier, the frequency slot, the time slot, and a wireless node weight.
 20. The computer readable storage medium of claim 19, wherein the specified channel access factor comprises a highest hash function value. 